Other
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt.........4o&link_type=abstract
Thesis (PHD). BRANDEIS UNIVERSITY , Source DAI-B 59/04, p. 1695, Oct 1998, 263 pages.
Other
2
Active Galactic Nuclei, Very Long Baseline Array
Scientific paper
The polarization and total intensity properties of a sample of blazars were monitored at two month intervals in order to determine the timescale of changes in the milliarcsecond polarization structure, obtain accurate proper motions, determine the location of polarization changes and shed light on the putative difference in polarization properties between quasars and BL Lacertae objects. The observations were made with the Very Long Baseline Array (VLBA), and the VLA. After correlation, the data were calibrated using the NRAO's AIPS software package and imaged and modelled using AIPS as well as Caltech's DIFMAP package. All the observed sources show significant structural changes on two month timescales and even shorter timescales are suggested by the data. Changes occur in one or more of the quantities: total intensity distribution, linear polarization and polarization position angle. Change in any one of these quantities is not necessarily linked to change in the others. Several sources also show more gradual changes on longer timescales. For one group of sources virtually all variation is located in the core. For the rest, jet components also show rapid variation. This may result from proximity to the central engine and, in a couple of cases, from a complex, poorly collimated jet. All eight sources for which proper motion could be reliably determined were superluminal. Six had previously reported proper motions from sparser measurements. All six numbers were consistent with our values, again suggesting there is little variation in the jet. In three sources that have multiple components with measured proper motion, the innermost component is significantly slower, suggesting an acceleration at the base of the jet. We detect low polarization in quasar cores and slightly higher values for the BL Lac cores. Rapid changes in position angle and polarized intensity are seen. The BL Lac/Quasar jet polarization difference is not seen close to the core but seems to appear further out along the jet. Finally, one source shows dramatic evidence of the existence of a 'sheath' with polarization different from that of the inner jet.
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